I work now but was a SAHM for 5 years.
Get out of the house with a fairly regular routine. Kids will enjoy it, and you will avoid going stir crazy.
Second the motion about going to the library. See if you have a local toy library as these are also great.
Find out if the local indoor play places do a M & T discount time, cheaper for you, and company too.
Take them for 'coffee and a biscuit' when you go shopping at Tescos etc! My kids loved the excitement of a biscuit in a cafe, and it is nice to have a cuppa made by someone else!
Have a stock of sticker books, colouring in stuff for when you need 10 minutes peace and quiet.
If you have NT properies near you with nice parkland look into getting a family ticket, pays for itself after about 4 visits.
Deff get the kids in bed by 7, this kept me sane, especialy since dh was away for months at a time so I was 'solo' SAHM for half of the time.